The present disclosure relates to image forming apparatuses.
Image forming apparatuses that use electrophotography are commonly known.
An electrophotographic image forming apparatus typically forms an image on paper by performing processes such as charging, light exposure, development, and transfer.
In a more specific example, an image bearing surface is uniformly charged to a specific electrical potential of a first polarity (for example, positive polarity) in a charging process. The image bearing surface is an outer circumferential surface of a photosensitive drum that acts as an image bearing member. Note that in the following explanation, the first polarity is positive polarity. Next, the image bearing surface is exposed to light in a light exposure process so as to form an electrostatic latent image on the image bearing surface. After the light exposure process, the electrostatic latent image on the image bearing surface is developed in a development process using charged toner. A toner image is formed on the image bearing surface through the development process. In a situation in which development is performed by reversal development, toner charged to the same polarity (first polarity) as the image bearing surface is used.
After the development process, a transfer bias of a second polarity is supplied to a transfer roller pressed against the photosensitive drum in a transfer process. The second polarity is the opposite polarity to the first polarity. The toner image formed on the image bearing surface is transferred onto paper as the paper passes through a nip between the photosensitive drum and the transfer roller. An image is formed on the paper through the transfer process. After the transfer process, cleaning and static elimination are performed on the image bearing surface before starting a next charging process.
Electrophotographic image forming apparatuses suffer from problems such as described below. The following problem is caused by the transfer bias. Upon the transfer bias of the second polarity being supplied to the transfer roller during the transfer process, current (transfer current) flows between the photosensitive drum and the transfer roller via the nip. As a result, the electrical potential of the image bearing surface, which is charged to the first polarity, decreases. In such a situation, a greater transfer current flows in a non-toner bearing section than in a toner bearing section. The term non-toner bearing section refers to a section of the image bearing surface on which toner is not present and the term toner bearing section refers to a section of the image bearing surface on which toner is present. Therefore, the non-toner bearing section has a lower electrical potential than the toner bearing section. In other words, non-uniformity arises in the electrical potential of the image bearing surface (non-uniform electrical potential). Furthermore, a situation may occur in which a section of the image bearing surface in which a particularly large transfer current flows becomes charged to the second polarity.
In a situation in which non-uniform electrical potential of the image bearing surface arises and a section of the image bearing surface becomes charged to the second polarity, electrical charge of the second polarity remains on the image bearing surface without being removed even if static elimination is performed in order to eliminate electrical charge of the first polarity and lower electrical potential of the image bearing surface to an initial electrical potential. Therefore, non-uniform electrical potential of the image bearing surface remains. As a consequence of the remaining non-uniform electrical potential, an image defect such as non-uniform image density may occur the next time that image formation is performed (in particular, when image formation is performed repeatedly with short intervals between repetitions).
Non-uniform electrical potential of the image bearing surface may also arise for the following reason. Electrical resistance of the nip is greater when paper is present in the nip than when paper is not present in the nip. Therefore, when an end of a sheet of paper in terms of a conveyance direction thereof (i.e., a leading end or a trailing end) passes through the nip there is a sudden change in the electrical resistance of the nip, which is accompanied by a sudden change in the transfer current. As a consequence, an end contact section of the image bearing surface exhibits a large fluctuation in electrical potential. The term end contact section refers to a section of the image bearing surface that has come into contact with the end of the sheet of paper in terms of the conveyance direction. As a result of the above, non-uniform electrical potential of the image bearing surface arises in a line-shaped pattern. For example, when a trailing end of a sheet of paper in terms of the conveyance direction passes through the nip, there is a sudden decrease in electrical resistance of the nip, which is accompanied by a sudden increase in the transfer current. As a consequence, the electrical potential of a trailing end contact section of the image bearing surface exhibits a large reduction in electrical potential. The term trailing end contact section refers to a section of the image bearing surface that has come into contact with the trailing end of the sheet of paper in terms of the conveyance direction. As a result of the above, non-uniform electrical potential of the image bearing surface arises in a line-shaped pattern. If the line-shaped non-uniform electrical potential remains on the image bearing surface, the next time image formation is performed an image defect appears as a horizontal line in a formed image.
One known example of an image forming apparatus includes a primary pre-charging device that is located downstream of a static elimination device and upstream of a primary charging device in terms of a rotation direction of an image bearing member.